1. Field of the Invention
The present invention relates to a method for driving a ferroelectric memory device, and more particularly to a method for driving a non-volatile semiconductor memory device in which a memory cell for storing data has a capacitor insulating film made from a ferroelectric material.
2. Description of the Related Art
An EPROM and an EEPROM have been put to practical use as a non-volatile memory device among semiconductor memory devices, and used for various kinds of personal computers and information apparatus.
A memory device using the EPROM, the EEPROM, or the like needs a time period for reading data the same as that of a RAM (a DRAM or a SRAM); however, the memory device needs a time period for writing or erasing data which is much longer than that of the RAM by some orders.
The EPROM should be irradiated with ultra violet rays in order to erase data. Thus, it is not easy to incorporate the EPROM into a device. Further, the EPROM should be wrapped with an expensive ceramic package having a quartz window through which ultra violet rays are irradiated. On the other hand, the EEPROM needs a higher voltage or a negative voltage for writing and erasing data. Thus, the EEPROM involves the inconvenience that another power supply is required for writing and erasing data, apart from a power supply for driving the memory device.
In order to solve these problems, a ferroelectric memory device in which a memory cell capacitor has an insulating film made of a ferroelectric substance film has been proposed. FIG. 11 shows a ferroelectric memory device in accordance with a conventional technique (the extended drafts of the International Solid-State Circuits Conference (ISSCC), 1989, p. 242). In this conventional memory device, each memory cell includes two ferroelectric capacitors and two switching transistors. A junction point of two ferroelectric capacitors is connected to a drive line. The memory device is driven by a so-called destructive reading method, and reading/writing data is accompanied with polarization inversion in the ferroelectric film of the capacitor.
When data is written into the memory cell, a + signal and a - signal are given to respective bit lines of each pair of bit lines and the two ferroelectric capacitors are polarized in the opposite direction, respectively. When data is read out from the memory cell, the bit lines are grounded and a pulse voltage is applied to the drive lines. Then, due to the difference in polarization directions of the capacitor insulating films, i.e. whether polarization inversion is caused or not, the potential difference is caused between the bit lines. The potential difference is amplified by a sense amplifier and a signal corresponding to stored data is detected.
In the above-mentioned conventional technique, two ferroelectric capacitors and two switching transistors are required for each memory cell (i.e. 1 bit), so that a chip area of 1 bit memory cell becomes quite large. In order to improve cost competitiveness, however, the chip area should be substantially equivalent to that of the EEPROM, the DRAM, and the like. For this purpose, a 1 capacitor-- 1 transistor structure is required for 1 memory cell.
Further, in the conventional technique, due to the destructive-reading, the possible number of times for reading-out from the memory cell is restricted by the operative life time of the ferroelectric film, i.e. the possible number of times for polarization inversion to obtain sufficient polarization. In current ferroelectric materials and process techniques, the operative life time of the ferroelectric film is in the range of 10.sup.9 to 10.sup.11 times. However, polarization inversion of 10.sup.15 times or more is required in order to use the device for 10 years. Thus, in the conventional techniques, the ferroelectric memory device cannot be available for practical use with a practical operative life time. Accordingly, it is preferable to use a method without polarization inversion for ordinary read/write operations.